Java Loops

Introduction

In programming, we often need to repeat a task multiple times.

For example, you may want to:

Print progress for 7 days
Check all completed modules
Repeat a menu until the user exits
Process multiple test cases

If you write the same statement repeatedly, the code becomes long and difficult to maintain.

This is where loops help.

Loops allow us to execute a block of code repeatedly.

What is a Loop in Java?

A loop is a control structure that repeats a block of code as long as a condition is satisfied.

Java provides three primary loops:

for loop
while loop
do-while loop

Each loop is useful in different situations.

Why Do We Need Loops?

Loops help in:

Reducing repetition
Writing shorter code
Solving pattern problems
Processing arrays and strings
Implementing DSA logic
Handling repeated input/output

Without loops:

System.out.println(1);
System.out.println(2);
System.out.println(3);
System.out.println(4);
System.out.println(5);

This approach is not scalable.

Using a loop is much better.

Java for Loop

The for loop is used when the number of iterations is known in advance.

Syntax

for (initialization; condition; update) {
    // code
}

Components of a for Loop

1. Initialization

Runs only once at the beginning.

int i = 1;

2. Condition

Checked before every iteration.

If true → loop executes
If false → loop stops

3. Update

Updates the loop variable after each iteration.

i++;

Example 1: Print Numbers from 1 to 5

public class Main {
    public static void main(String[] args) {

        for (int day = 1; day <= 5; day++) {
            System.out.println(day);
        }
    }
}

Output

Dry Run

Iteration	day	Condition	Output
1	1	True	1
2	2	True	2
3	3	True	3
4	4	True	4
5	5	True	5
6	6	False	Stop

Example 2: Print Even Numbers from 2 to 10

for (int value = 2; value <= 10; value += 2) {
    System.out.println(value);
}

Output

Example 3: Print a Message 3 Times

for (int attempt = 1; attempt <= 3; attempt++) {
    System.out.println("Keep practicing");
}

When to Use for Loop?

Use a for loop when:

Number of iterations is known
Working with indices
Traversing arrays
Solving pattern problems

Flow of a for Loop

Initialization
      ↓
Condition Check
      ↓
Loop Body
      ↓
Update
      ↓
Condition Check Again

Repeats until the condition becomes false.

Java while Loop

The while loop is used when the number of iterations is not known beforehand.

Syntax

while (condition) {
    // code
}

Example 4: Print Numbers from 1 to 5

public class Main {
    public static void main(String[] args) {

        int count = 1;

        while (count <= 5) {
            System.out.println(count);
            count++;
        }
    }
}

Output

Example 5: Countdown

public class Main {
    public static void main(String[] args) {

        int modulesLeft = 4;

        while (modulesLeft >= 1) {
            System.out.println("Modules left: " + modulesLeft);
            modulesLeft--;
        }
    }
}

Output

Modules left: 4
Modules left: 3
Modules left: 2
Modules left: 1

When to Use while Loop?

Use a while loop when:

Iterations are unknown
Waiting for user input
Loop depends on a condition
Processing until something becomes false

Java do-while Loop

The do-while loop executes the body at least once.

Syntax

do {
    // code
} while (condition);

Notice the semicolon after while(condition).

Example 6: Revision Rounds

public class Main {
    public static void main(String[] args) {

        int revisionRound = 1;

        do {
            System.out.println("Revision Round: " + revisionRound);
            revisionRound++;
        } while (revisionRound <= 3);
    }
}

Output

Revision Round: 1
Revision Round: 2
Revision Round: 3

Example 7: Runs At Least Once

public class Main {
    public static void main(String[] args) {

        int score = 10;

        do {
            System.out.println("This runs once.");
        } while (score > 50);
    }
}

Output

This runs once.

Even though the condition is false, the loop body executes once.

while vs do-while

Feature	while	do-while
Condition Checked	Before execution	After execution
Minimum Executions	0	1
Use Case	Condition-based loops	Menus and prompts

Example Comparison

while

int x = 10;

while (x > 20) {
    System.out.println("Hello");
}

Output:

No Output

do-while

int x = 10;

do {
    System.out.println("Hello");
} while (x > 20);

Output:

Hello

Enhanced for Loop (For-Each Loop)

Used when you only need the values and not the index.

Syntax

for (datatype variable : collection) {
    // code
}

Example

int[] nums = {10, 20, 30};

for (int num : nums) {
    System.out.println(num);
}

Output

10
20
30

When to Use?

Traversing arrays
Traversing collections
When index is not required

Infinite Loops

If a loop condition never becomes false, the loop runs forever.

This is called an infinite loop.

Example 8: Infinite while Loop

while (true) {
    System.out.println("This keeps running forever");
}

Example 9: Infinite for Loop

for (;;) {
    System.out.println("Infinite loop");
}

Common Causes

Missing update statement
Incorrect condition
Variable not changing

Example:

int num = 1;

while (num <= 5) {
    System.out.println(num);
}

This loop never ends because num++ is missing.

Nested Loops

A loop inside another loop is called a nested loop.

Used in:

Pattern problems
Matrices
Grids
Table generation

Example 10: Square Pattern

public class Main {
    public static void main(String[] args) {

        for (int row = 1; row <= 3; row++) {

            for (int col = 1; col <= 3; col++) {
                System.out.print("* ");
            }

            System.out.println();
        }
    }
}

Output

* * *
* * *
* * *

Understanding

Outer loop → Rows
Inner loop → Columns

Loop Control Statements

Java provides:

break
continue

break Statement

Stops the loop immediately.

Example 11

for (int testCase = 1; testCase <= 6; testCase++) {

    if (testCase == 4) {
        break;
    }

    System.out.println(testCase);
}

Output

1
2
3

continue Statement

Skips the current iteration and moves to the next one.

Example 12

for (int day = 1; day <= 5; day++) {

    if (day == 3) {
        continue;
    }

    System.out.println(day);
}

Output

Loop Selection Cheat Sheet

Situation	Loop
Known iterations	for
Unknown iterations	while
Must execute once	do-while
Pattern problems	for
Array traversal	for
Menu systems	do-while

Loops and Complexity

Loops are closely related to Time Complexity.

Single Loop

for (int i = 0; i < n; i++)

Time Complexity:

O(n)

Nested Loop

for (int i = 0; i < n; i++)
    for (int j = 0; j < n; j++)

Time Complexity:

O(n²)

Halving Loop

for (int i = n; i > 1; i /= 2)

Time Complexity:

O(log n)

Common Beginner Mistakes

1. Forgetting Update

int i = 1;

while (i <= 5) {
    System.out.println(i);
}

Results in an infinite loop.

2. Wrong Condition

for (int i = 1; i >= 5; i++) {
    System.out.println(i);
}

Loop never runs.

3. Extra Semicolon

for (int i = 1; i <= 5; i++);
{
    System.out.println("Hello");
}

The semicolon ends the loop.

4. Confusing while and do-while

Remember:

while    → checks first
do-while → runs first

5. Poor Variable Names

Prefer:

row
col
i
j
index
count

instead of unclear names.

Interview and DSA Relevance

Loops are used in:

Arrays
Strings
Searching
Sorting
Pattern Problems
Dynamic Programming
Matrix Traversal

Strong loop fundamentals make DSA significantly easier.

Practice Questions

Print numbers from 1 to 10 using a for loop.
Print numbers from 10 to 1 using a while loop.
Print the first 5 even numbers.
Print the multiplication table of 7.
Create a menu using do-while.
Find the sum of numbers from 1 to n.
Print an n × n square pattern.
Print odd numbers from 1 to 20 using continue.
Stop a loop when the value becomes 6 using break.

Quick Summary

Java provides three primary loops:

for → Fixed repetitions
while → Condition-based repetitions
do-while → At least one execution

Also remember:

break stops a loop
continue skips an iteration
Nested loops are used for patterns and grids
Missing updates can create infinite loops

Final Takeaway

Loops are not just another Java topic.

They are one of the strongest foundations of programming.

If conditionals teach your program how to decide,

then loops teach your program how to repeat logic efficiently.

This is why loops appear everywhere:

Beginner programs
Pattern problems
DSA
Real-world software systems

Mastering loops forms the bridge between:

Basic Syntax
      ↓
Logical Programming
      ↓
Problem Solving
      ↓
Data Structures & Algorithms

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Java Loops

Introduction

What is a Loop in Java?

Why Do We Need Loops?

Java for Loop

Syntax

Components of a for Loop

1. Initialization

2. Condition

3. Update

Example 1: Print Numbers from 1 to 5

Output

Dry Run

Example 2: Print Even Numbers from 2 to 10

Output

Example 3: Print a Message 3 Times

When to Use for Loop?

Flow of a for Loop

Java while Loop

Syntax

Example 4: Print Numbers from 1 to 5

Output

Example 5: Countdown

Output

When to Use while Loop?

Java do-while Loop

Syntax

Example 6: Revision Rounds

Output

Example 7: Runs At Least Once

Output

while vs do-while

Example Comparison

while

do-while

Enhanced for Loop (For-Each Loop)

Syntax

Example

Output

When to Use?

Infinite Loops

Example 8: Infinite while Loop

Example 9: Infinite for Loop

Common Causes

Nested Loops

Example 10: Square Pattern

Output

Understanding

Loop Control Statements

break Statement

Example 11

Output

continue Statement

Example 12

Output

Loop Selection Cheat Sheet

Loops and Complexity

Single Loop

Nested Loop

Halving Loop

Common Beginner Mistakes

1. Forgetting Update

2. Wrong Condition

3. Extra Semicolon

4. Confusing while and do-while

5. Poor Variable Names

Interview and DSA Relevance

Practice Questions

Quick Summary

Final Takeaway

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